/Engine/Source/Editor/UnrealEd/Classes/Editor/UnrealEdEngine.h
Generate the package thumbails if they are needed.
[UPackage](API\Runtime\CoreUObject\UObject\UPackage) * GeneratePackageThumbnailsIfRequired ( const TCHAR * Str, [FOutputDevice](API\Runtime\Core\Misc\FOutputDevice) & Ar, [TArray](API\Runtime\Core\Containers\TArray)< [FString](API\Runtime\Core\Containers\FString) > & ThumbNamesToUnload )